The invention relates to a process by means of which a catalytic coating can be introduced into the pores of a ceramic flow-through honeycomb body using a catalyst suspension comprising catalyst components as solids and/or in dissolved form in a carrier liquid. The coated honeycomb bodies are preferably used for the purification of automobile exhaust gases.
Catalysts applied in the form of a coating to honeycomb bodies have for decades been used for the purification of automobile exhaust gases. Parallel flow channels for the exhaust gases run through the honeycomb bodies. Ceramic honeycomb bodies are produced from refractory materials. The material is predominantly cordierite, a magnesium-aluminum silicate. Further customary ceramic materials are silicon carbide, aluminum oxide, zirconium oxide, zirconia-mullite, mullite, silicon nitride, aluminum titanate and titanium oxide. The honeycomb bodies are produced from these materials by extrusion and generally have an open pore structure.
The flow channels run through the honeycomb bodies from the entry end face to the exit end face. The channels generally have a square cross section and are arranged in a dense grid pattern over the cross section of the honeycomb bodies. The number of flow channels per unit cross-sectional area is referred to as the cell density and can be in the range from 10 to 200 cm−2.
The catalytic coating of the honeycomb bodies is a dispersion coating which is applied to the honeycomb bodies using a usually aqueous suspension of the catalyst components. This coating is frequently also referred to as a washcoat.
The catalyst components comprise, for example, pulverulent support materials having a high specific surface area onto which the catalytically active components, usually the noble metals of the platinum group, platinum, palladium, rhodium, iridium and ruthenium are applied. The solids in the catalyst suspension are generally homogenized by wet milling before being applied to the honeycomb bodies. After milling, the solids of the suspension have an average particle size d50 in the range from 3 to 5 μm.
Examples of support materials are simple and composite oxides, e.g. active aluminum oxide, zirconium oxide, tin oxide, cerium oxide or other rare earth oxides, silicon oxide, titanium oxide or silicates such as aluminum silicate or titanates such as barium or aluminum titanate and zeolites. The various phases of active transition aluminum oxide which can be stabilized by doping with silicon oxide and lanthanum oxide and also by zirconium oxide and cerium oxide have been found to be particularly useful as heat-resistant support materials.
The catalytic activity and aging stability of the finished catalyst is generally greater the greater the concentration of the catalytic composition on the honeycomb body. In practice, from 10 to 300 g/l are required, depending on the application. However, the maximum achievable concentration can be below the catalytically required concentration for various reasons. Thus, the adhesion of the coating decreases with increasing concentration and thus layer thickness. In addition, high layer thicknesses reduce the hydraulic diameter of the flow channels and thus increase the counter pressure of the exhaust gas (banking-up pressure) through the catalyst.
There are fields of application, for example the oxidation of hydrocarbons and carbon monoxide in diesel exhaust gas (“diesel oxidation catalyst”), in which only a relatively low mass of catalyst in the range from 100 to 200 g per liter of honeycomb body volume is necessary for the reaction. A further increase in the mass of catalyst while maintaining the total noble metal content is not associated with any activity advantage in this case. In other catalytic reactions, for example the storage and reduction of nitrogen oxides (“nitrogen oxide storage catalyst”) or the selective catalytic reduction of nitrogen oxides by means of ammonia (“SCR catalyst”), on the other hand, an increase in the active mass is desirable but, owing to the above-mentioned problems with adhesion of the coating and the banking-up pressure through the finished catalyst, is possible only within limits.
U.S. Pat. No. 5,334,570 proposes reducing the high banking-up pressure by relocating the catalytic coating into the pores of ceramic honeycomb bodies. The ceramic honeycomb bodies used in this patent had an open porosity of from 30 to 45% and an average pore diameter of from 3 to 10 μm. Catalyst materials which have colloidal particle diameters in the range from 0.001 to 0.1, preferably from 0.001 to 0.05 μm, and on contact of the honeycomb bodies with a corresponding colloidal coating suspension penetrate into the pores of the honeycomb bodies were therefore selected for catalytic coatings. According to the patent, the honeycomb bodies were contacted with the coating suspension by dipping them into the coating suspension. In this way, from 90 to 95% of the colloidal washcoat particles could be introduced into the pores of the honeycomb bodies so that the cross section of the flow channels was barely reduced by the coating and the banking-up pressure was thus increased only inconsequentially.
In recent years, ceramic honeycomb bodies having a significantly increased porosity of about 60-65% and average pore diameters of from 10 to 20 μm have been developed. The objective here was to make the channel walls permeable to the catalyst particles, so that the latter could deposit not only as a layer on the channel surface but also in the pore system of the wall. In this way, lower layer thicknesses at a comparable catalyst mass or, conversely, higher loading concentrations at the same catalyst layer thickness can be achieved [Tao et al., SAE 2004-01-1293].
To coat honeycomb bodies, the catalytically active, water-insoluble, pulverulent components are usually suspended in water or an organic liquid, milled and the substrate is subsequently coated by dipping into the suspension, by pouring the suspension over the substrate or by drawing-in or pumping-in of the suspension.
If use is made here of the above-described, newly developed porous honeycomb bodies, part of the catalytically active substances actually penetrate into the pore system of the honeycomb body and is deposited there. However, this increases the amount of catalyst deposited on the honeycomb body to only a small extent.